1. Field Of The Invention
The present invention relates to a system and method for encoding color signals while minimizing visible artifacts in the displayed image, and more particularly, to a system and method for jointly encoding the respective color intensity signals so as to minimize visible artifacts, particularly those caused by errors in the luminance signal due to the color signal encoding.
2. Description Of The Prior Art
Color encoders are widely used in computer graphics and color television display systems for encoding the color signals to be displayed on the display screen. FIG. 1 illustrates a simplified prior art computer graphics display system in which the respective green, red and blue digitized color signals are respectively interpolated by interpolation circuits 10G, 10R and 10B for computing the color values at the respective pixels of the display device. These interpolated values are then input into an encoder 20 where they are separately encoded for storage in respective frame buffers 30G, 30R and 30B. In some display systems, these encoded color values are then used as indices into color look-up tables or color maps 40G, 40R and 40B for determining the actual color to be displayed on the display screen. The resulting color values read from the color maps 40G, 40R and 40B are then converted to analog voltages by respective digital to analog converters 50G, 50R and 50B, and these analog voltages are applied to a color monitor 60 for driving the electron guns of the color monitor 60 in accordance with known techniques.
A very simple prior art encoder for use as encoder 20 in the computer graphics display system described above with respect to FIG. 1 is represented as 20A in FIG. 2. Such a prior art encoder 20A simply truncates an input color intensity signal having a predetermined number of digital bits (typically 10-16 bits) to a lesser number of high order bits (typically 4-8 bits), and these high order bits are then stored as the encoded values in the respective frame buffers 30G, 30R and 30B. The low order bits are generally discarded. A truncation circuit 20A is generally provided in each color channel to separately truncate the respective color values (R, G and B). Such a simple color encoder has the benefits that it is easy to implement and minimizes memory usage in the frame buffers. However, by so limiting the encoded values to the high order bits, the accuracy of the encoded color values is adversely affected.
FIG. 3 illustrates another prior art encoder 20B for use as encoder 20 in the computer graphics display system of FIG. 1. The circuit of FIG. 3 improves the color resolution by utilizing dithered encoding of each of the color values separately. This technique has the benefit that although the encoded output signal is limited to a predetermined number of high order bits (N), the encoded output signal has an average of N+2 bits of color accuracy. This is accomplished by taking the high order N bits of the input color fraction signal (R, G or B) and applying it to an adder 30 along with dither bits N+1 and N+2. These dither bits are added to the least significant bits of the X and Y pixel addresses for determining dither values which are added to the high order N bits in adder 30. The summed values thus change as the pixel addresses change from one point to the next. The low order bits of the input color fraction signal are generally discarded (as in the case of truncation), and the useful output fraction of the adder 30 is limited to the dithered N bits. When dither encoding each color value separately in such an embodiment, the input intensity value must be scaled not to exceed all ones in the output fraction so that adder 30 will not overflow. Such an embodiment is beneficial because when a constant colored region is so encoded, groups of four adjacent pixels will average N+2 bits of color accuracy rather than N bits as for the encoder 20A of FIG. 2.
However, besides their inherent limitation in color accuracy because of their truncation of the input color intensity values, the prior art encoders 20A and 20B of FIGS. 2 and 3 have another major limitation. Namely, by discarding the low order bits for each of the color intensity signals, visible artifacts are introduced into the displayed image. These artifacts generally occur in the luminance (Y) signal, which, as known to those skilled in the art, is a weighted sum of the red, green and blue color intensity signals and is typically represented as a fraction having a value between 0 and 1. If the luminance signal has a value of 1, it represents a fully saturated white, while if the luminance signal has a value less than 1, some other color is represented. Since the luminance signal is comprised of fractional portions of the red, green and blue signals, it is adversely affected by the truncation of each of these color signals during encoding. Such effects on the luminance signal are very undesirable, for the human eye is more sensitive to changes in the luminance values of a displayed image than changes in the color intensity signals. Accordingly, such discarding of lower order bit information for each of the color signals adversely affects the luminance detail of the image displayed, thereby causing visible artifacts noticeable to the viewer.
Accordingly, it is desired to develop an encoding system and method for use in a computer color graphics or color television display system whereby the visible artifacts introduced into the displayed image by the encoding of the color intensity signals are minimized without unduly increasing system cost. The present invention has been designed to solve this problem.